A patient who developed cerebellitis following infectious mononucleosis was found to have oligoclonal IgG bands in cerebrospinal fluid but not serum during active phases of her disease. Some of the antibody in two of these bands was directed against Epstein-Barr virus capsid antigen. Active Epstein-Barr virus infection during the course of her neurological disease was demonstrated by establishment of Epstein-Barr virus containing lymphoid cell lines from biopsies and elevated serologic responses. The ability to infect a bone marrow recipient with EBV from a donor who contacted infectious mononucleosis was demonstrated.